1. Field of the Invention
The instant invention relates to making a usable powdered fuel from municipal or other solid waste sources by embrittling the solid organic fraction of said waste at elevated temperatures and in the presence of an organic acid, e.g. oxalic acid, citric acid or maleic acid. After embrittlement, the embrittled solid organic fraction is comminuted, e.g. by grinding, to less than a predetermined particle size and a powdered fuel having a high caloric value is recovered. The powdered fuel may be used as is or can be cofired with other fuels such as heating oils or coal.
2. Description of the Prior Art
Many processes for the reclamation of various valuable materials from waste sources are known. For example, municipal solid waste may be a source of aluminum, ferrous metals, glass, plastics, paper and textiles.
In an exemplary process for the separation of valuable materials from solid waste sources, the incoming waste is shredded to reduce the waste to easily handled particle size, e.g. less than 4 inches. The shredded waste may be passed through a magnetic field to remove ferrous metals and subjected to air-classification or other procedures known in the art to separate the lighter materials, such as the various organics present in the waste, from the heavier materials such as glass, rock, dirt, etc. The heavier materials may be further treated to remove the electroconductive, non-magnetic metals, e.g. aluminum fragments, by processes that rely on the electroconductive nature of aluminum to separate it from admixture with other materials. The glass may be recovered by procedures known in the art such as froth flotation. Finally, the lighter organic materials, e.g. the overhead from the air-classification may be burned to provide heat or pyrolyzed to provide gaseous or liquid products. In general, the objective of municipal waste treatment processes is to recover all valuable materials and minimize the amount of said waste that must be disposed of as landfill.
In a process described in U.S. Pat. No. 3,961,913 and No. 4,008,053, the solid organic fraction of municipal waste or other waste is converted into a powdered fuel. In the process described in these patents a mineral acid is used to embrittle the organic fraction and said embrittled organic fraction is then ground to a powdered fuel. The difficulty inherent in this process is that the mineral acids such as hydrochloric acid or sulphuric acid, while efficient for embrittling the solid organic fraction, leave various undesirable moieties in the powdered fuel. For example, fuel derived from the solid organic fraction of municipal solid waste is, in and of itself, very low in sulphur. When sulphuric acid, however, is utilized as the embrittling agent in the above process, significant amounts of sulphur remain in the powdered fuel that is recovered. It is known in the art that the burning of sulphur containing fuels is undesirable from an environmental standpoint.
The use of hydrochloric acid as the embrittling agent results in a significant amount of chlorine in the powdered fuel recovered from the process. Hydrochloric acid causes corrosion problems, during the embrittlement process as well as during the burning of the powdered fuel derived from the hydrochloric acid embrittlement process. In order to use the above powdered fuels, additional treatment of the fuel to remove sulphur or chlorine is required prior to or during the combustion process to prevent these undesirables from entering the environment. These additional requirements, of course, decrease the value of the solid fuel made by the process described in the above patents.
Another reference which suggests the use of chemical embrittling agents to facilitate the conversion of the solid organic fraction of municipal solid waste into a powdered fuel is EPA-600/7-78-143, authored by N. L. Hecht, et al. and entitled "Investigation of Advanced Thermal-Chemical Concepts for Obtaining Improved MSW-Derived Products", August, 1978. The authors in this report tested formaldehyde, hydrochloric acid, chlorine, sulphur dioxide, etc. as chemical embrittling agents. In addition, experiments involving the use of formic acid and acetic acid with formaldehyde were run but the authors concluded that these combinations failed to produce "significant embrittlement". Finally, the authors did not disclose the particle size of the powdered fuel which they obtained in any of their experiments.
The above problems associated with the prior art processes for preparing powdered fuel from municipal and other waste sources are overcome by the method of the instant invention.